I don't remember where I read this but the police did extensive work in the schools in many states to spot any girl who would have stop going to school or changed school suddenly, that kind of thing. They worked every suspect case until they found the maching kid. It gaves nothing at all, no kid that was unaccounted for. That is what is very weird in this case: a kid that age leaves traces at school, the doctor, the dentist, her friends, whatever but nothing at all surfaced. Maybe she was kept apart from the begining, like some other kidnaped children.
I remember reading that, but IIRC they searched schools within a certain radius of St. Louis (like 100 miles or something). She could have been from anywhere. California, Washington, Maine, Florida...even Alaska or Hawaii.